This invention relates to a circuit breaker mechanism.
In certain applications, it may be desirable for a circuit breaker to have an instantaneous tripping current which is relatively high, due to the type of load controlled by the circuit breaker. For example, when certain electric motors are started, they draw a high inrush current, which quickly reduces to a substantially lower operating current. A conventional circuit breaker, correctly rated according to the normal operating current of the motor, may trip due to the high inrush current when the motor is started, which is obviously undesirable, and may lead to the fitting of an unsuitably highly rated circuit breaker to alleviate the problem.
It is an object of the invention to provide a circuit breaker mechanism with a relatively high instantaneous tripping current characteristic.
A circuit breaker mechanism comprising:
a coil arranged to carry a load current and defining an axis;
a magnetic circuit including a pole piece aligned with the axis of the coil and arranged to concentrate magnetic flux due to current in the coil;
an armature supported adjacent the coil and movable transversely relative to the axis of the coil, the armature having a head portion which is attracted towards the pole piece under the influence of magnetic flux in the pole piece, thereby generating an operating moment on the armature, and a foot portion which is attracted to a part of the magnetic circuit remote from the pole piece, thereby generating an opposing moment on the armature.
The magnetic circuit may include a magnetic element movable towards the pole piece along the axis of the coil against the urging of a bias element, the magnetic element having a rest position adjacent the foot portion of the armature so that the opposing moment is greater when the magnetic element is in the rest position.
The magnetic element may be a magnetic core movable against a bias element from the rest position towards the pole piece.
Preferably, the magnetic core is movable in a tube of non-magnetic material against the urging of a spring located in the tube between the magnetic core and the pole piece.
The armature may comprise a length of magnetic material mounted pivotably to the magnetic frame at a pivot point intermediate the head and foot portions of the armature.
The armature may be formed from steel sheet or bar.
The head and foot portions of the armature preferably extend transversely from the armature at respective opposed ends thereof.
The mechanism may include an adjuster comprising a spacer element locatable between the foot portion of the armature and the magnetic element, and movable to vary the distance between the foot position and the magnetic element, thereby to adjust the instantaneous tripping characteristic of the circuit breaker mechanism.
The spacer element may comprise a cylindrical body of magnetic material mounted for rotation about the axis of the coil, the body being eccentric so as to move the foot portion of the armature towards or away from the magnetic element as the body is rotated.
Preferably, the adjuster has a tool engaging formation accessible via an opening in a housing for the circuit breaker mechanism, to permit adjustment of the instantaneous tripping characteristic of the circuit breaker mechanism after installation thereof in use.